He needs the mask because he needs to inhale that medication 24/7. It's better to ask why he needs a mask when using Venom

Some villains just wear masks to look more intimidating. Look at Ledger's Joker. Did he have to wear facepaint to be a crazy criminal mastermind? No - but he did. Just because to frighten people as it was said in TDK.

Even in the Nolanverse not every outfit is just about being practical. Why does a cat burglar like Selina wear a domino mask and those unpractical heels? And Crane's fear gas also worked without his mask as the third act in Begins showed. The Nolanverse is not very consistent in this point.

And - it' still a comic book movie. Heroes and villains look this way in comic books. Superman could also fly without a blue suit, red trunks and a cape. And The Riddler on the other hand could ask riddles without a suit full of question marks. But they wear those things 'cause they are their "some kind of corporate design".